While they gain quite a bit of health (the d4d warthog, started with 3500hp, when i finished it was over 10000hp) the accuracy definatly goes down.
two of them firing side by side, the left one has over 10 experience, the right one is straight from the factory

Edit: I just did this with vulcans, and I didn't have enough time to get one of them up to 10, so i didn't see any drastic difference between a brand new one and an experienced one